Process for the separation of diene from organic mixtures

ABSTRACT

Dienes are separated from organic mixtures comprising diene and alkene having one double bond by contacting the mixture against one side of a glycidyl methacrylate polymer membrane and withdrawing at the other side a vaporous mixture having increased diene concentration. Exemplary of the organic mixtures is a mixture of butadiene and butene.

United States Patent Perry et al.

[ 51 Jan. 29, 1974 PROCESS FOR THE SEPARATION OF DIENE FROM ORGANICMIXTURES Inventors: Eli Perry, St. Louis; William F.

Strazik, St. Ann, both of Mo.

Assignee: Monsanto Company, St. Louis, Mo.

Filed: Sept. 22, 1972 Appl. No.: 291,460

US. Cl. 260/6815 R, 260/677 A Int. Cl. C07c 7/00 Field of Search...260/681.5 R, 677 A; 208/308 References Cited UNITED STATES PATENTS2/1968 Carpenter et al 208/308 Primary Examiner-Delbert E. GantzAssistant Examiner-Veronica OKeefe Attorney, Agent, or Firm-Lynden N.Goodwin et al.

[ 57] ABSTRACT 4 Claims, No Drawings PROCESS FOR THE SEPARATION OF DIENEFROM ORGANIC MIXTURES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to a process for separating dienes fromorganic mixtures containing same. In a particular aspect this inventionrelates to a process for the separation of diene from organic mixturescomprising diene and alkene having one double bond by preferentialpermeation through a membrane under pervaporation conditions. In a moreparticular aspect this invention relates to a process for the separationof diene from an organic mixture comprising diene and alkene having onedouble bond by contacting said mixture against one side of aglycidylmethacrylate polymer membrane and recovering on the other side avaporous mixture rich in diene.

2. Description of the Prior Art Processes for the preparation of dienessuch as butadiene and isoprene yield reaction mixtures which containorganic reaction products (typically substituted and unsubstituted C Chydrocarbons) in addition to organic solvents and the desired diene.Separation of dienes from such organic reaction media has beenaccomplished by distillation procedures. Principally because of theclose boiling points of dienes and typical reaction by-products,especially the corresponding alkenes having one double bond high refluxratios or azeotropic agents and costly distillation equipment arerequired for the distillation separation procedure.

Separation of components of azeotropic mixtures of organic materials bypervaporation through polymer membranes followed by distillation isknown to the art from the US. Pat. No. 2,953,502 issued Sept. 20, 1960to R. C. Binning and Robert J. Lee.

SUMMARY OF THE INVENTION It has been discovered in accordance with thepresent invention that dienes are effectively separated from organicmixtures comprising diene and alkene having one double bond bycontacting the mixture against one side of a glycidyl methacrylatepolymer membrane and withdrawing at the second side a vaporous mixturehaving a higher concentration of diene than the aforesaid mixture.Glycidyl methacrylate polymer membranes employed in the process of thepresent invention are highly efficient in separating diene from othercomponents of such organic mixtures using pervaporation separationtechniques. The present invention is further advantageous in that itpermits avoidance of costly distillation procedures.

DETAILED DESCRIPTION The process of the present invention comprisescontacting an organic feed mixture comprising diene and alkene havingone double bond against one side of a membrane comprising glycidylmethacrylate polymer and withdrawing at the second side a mixture havinga higher concentration of the preferentially permeable diene than theaforesaid feed mixture. It is essential that the mixture at the secondside be maintained at a lower chemical potential than that on the feedside. It is also essential that the product be withdrawn at the secondside in the vapor state. In the commercial utilization of the process,multi-stage operation is feasible since this permits the operation ofindividual stages at various concentrations and temperatures in order toachieve the optimum driving force for the process.

For each individual stage the effectiveness of the separation is shownby the separation factor (S.F.).

The separation factor (S.F.) is defined as the ratio of theconcentrations of two substances, A and B, to be separated, divided intothe ratio of the concentrations of the corresponding substances in thepermeate,

SE. (C /C in permeate (C /C in permeant where C and C are theconcentrations of the preferentially permeable component and any othercomponent of the mixture or of the sum of other components respectively.

In carrying the process of the present invention, the first or feed sideof the membrane is such that the activities of the components aregreater than the activities on the second side. Preferably the firstside is above atmospheric pressure and the second side below atmosphericpressure. Still more preferably, the second side is maintained such thatthe pressure differential is greater than 0.01 atmosphere. A furtherpreferred mode of operation is with the second side maintained at avacuum of greater than 0.2 mm. Hg.

The term Chemical Potential is employed herein as described by Olaf A.Hougen and K. M. Watson (Chemical Process Principles, Part II, JohnWiley, New York, 1947). It is related to the escaping tendency of asubstance from any particular phase. For an ideal vapor or gas, thisescaping tendency is equal to the partial pressure. For a liquid, thechange in escaping tendency as a function of total pressure is small.The escaping tendency always depends upon the temperature andconcentration. In the present invention, the feed substance is typicallya liquid solution and the other side of the membrane is maintained suchthat a vapor phase exists. A vapor feed may be employed when the mixtureto be separated is available in that form from an industrial process orwhen heat economies are to be effected in a multi-stage process.

The feed side may be at pressures less than atmospheric, but preferablygreater than atmospheric and also at pressures over and above the vaporpressure of the liquid components. The collection or permeate vapor sideof the membrane is preferably less than atmospheric pressure, but underproper feed side conditions, also may be greater than atmosphericpressure. The total. pressure on the feed side is preferably between 0psi absolute and 5000 psig.

The conditions are always such as to maintain a higher chemicalpotential on the feed side than on the collection or vapor side.

The temperatures on the feed side and the collection side may vary overa wide range. I-Iowever, temperatures should be avoided which causesubstantial decomposition of any of the organic materials in the mixtureor of the membrane and which cause the vapor pressure on the collectionside of any of the permeated materials to be exceeded by the pressurebeing maintained on that side. Typically, an increase in temperaturecauses an increase in permeation rate. A dramatic increase in rate oftenoccurs when the temperature exceeds the glass transition temperature ofthe polymer membrane being used in the separation procedure.

The process of the present invention provides for the separation ofdiene from organic mixtures comprising diene and alkene having onedouble bond. Such dienes are substituted and unsubstituted and typicallycontain from four to eight carbon atoms. A diene may be substitutedwith, for example alkyl, aromatic and halogen substituents. Typicalorganic components and mixtures from which the dienes are separatedinclude C C alkenes such as butene, hexene, propylene, and heptene aswell as other hydrocarbons such as chlorohexane, acrylic acid, octane,propane, etc. and the like. Separations are carried out by removal ofthe preferentially permeable diene through the membrane with the saiddiene, in a higher concentration than in the feed being recovered fromthe collection side of the membrane as a vapor and with the impositionof a state of lower chemical potential on such collection side of themembrane. For example, a mixture of butadiene and butene may be appliedto one side of a flat diaphram or membrane to accomplish removal of atleast a part of the butadiene, leaving a more highly concentrated butenesolution at the feed side of the membrane or diaphram. A state of lowerchemical potential is maintained on the collection or downstream side ofthe membrane by vacuum e.g. from 0.1 mm Hg. to the vapor pressure of theorganic component of the mixture which has the lowest vapor pressure atthe membrane at the respective temperature as long as the vapor phase ispresent on the downstream side.

In the system referred to above, the butadiene selectively passesthrough the membrane with the butadiene-rich composition being rapidlyremoved as vapor from the collection side of the membrane.

In contrast to the present invention, the employment of permeates inliquid phase on the second side of the membrane is impractical becausethe applied pressure has been found to be prohibitively high, e.g. up to1,000 atmospheres being necessary because of osmotic pressures.Liquid-liquid permeation is largely an equilibrium phenomenon unless theosmotic forces are overcome while in contrast liquid-vapor orvapor-vapor permeations are rate controlled processes even at moderateconditions, in which the vapor is removed as soon as it reaches thecollection surface of the membrane. Liquid-vapor and vapor-vaporseparations are accordingly much more effectively carried out thanliquidliquid separations.

The permeation membrane used in the process of the present inventioncomprises glycidyl methacrylate polymers. The membrane may be a simpledisc or sheet of the membrane substance which is suitably mounted in aduct or pipe, or mounted in a plate and frame filter press. Other formsof membranes may also be employed such as hollow tubes and fibersthrough which or around which the feed is supplied or is recirculatedwith the product being removed at the other side of the tubes as avapor. Various other shapes and sizes are readily adaptable tocommercial installations. The membrane, of course, must be insoluble inthe organic separation medium. Membrane insolubility as used herein istaken to include that the membrane material is not substantiallysolution-swellable or sufficiently weakened by its presence in thesolution to impart rubbery characteristics which can cause creep andrupture under the conditions of use, including high pressures. Themolecular weight of the glycidyl methacrylate polymers may vary over awide range, but in all cases should be sufficient to permit the polymerto be formed into a film which is sufficiently strong to withstandseparation processing conditions.

The membranes may be prepared by any suitable procedure such as, forexample, by casting a film or spinning a hollow fiber from a dope"containing polymer and solvent. Such preparations are well known to theart.

An important control over the separation capacity of a particularmembrane is exercised by the method used to form and solidify themembrane (e.g. casting from a melt into controlled atmospheres or fromsolution into baths at various concentrations and temperatures).

The art of membrane usage is well known with substantial literaturebeing available on membrane support, fluid flow and the like. Thepresent invention is practiced with such conventional procedures andapparatus. The membrane must, of course, be sufficiently thin to permitpermeation as desired, but sufficiently thick so as to not rupture underthe pressure conditions employed. Typically suitable membranes have athickness of from about /2 to about 10 mils.

The following examples illustrate specific embodiments of the presentinvention. In the examples the membrane employed was in the form of filmdisks and was mounted in a membrane holder. The membrane which was 1 milin thickness was prepared by casting from solution.

EXAMPLE 1 EXAMPLE 2 The procedure of Example 1 is followed to separateisoprene from a liquid mixture of isoprene, hexene, and pentane using amembrane which comprises glycidyl methacrylate polymer. While theinvention has been described with reference to particular embodimentsthereof, it will be appreciated that modifications and variations arepossible without departing from the invention.

We claim:

1. A process for the separation of diene from an organic mixturecomprising diene and alkene having one double bond which comprisescontacting the said mixture against one side of a glycidyl methacrylatepolymer membrane and withdrawing at the second side of the membrane avaporous mixture having a higher concentration of diene than theaforesaid organic mixture with the mixture at the second side beingmaintained at a lower chemical potential than the mixture on the otherside of the membrane.

2. The process of claim 1 wherein the pressure on the second side of themembrane is less than atmospheric pressure and lower than the pressureon the other side of the membrane.

3. The process of claim 1 wherein the said organic mixture is a liquidmixture.

4. The process of claim 1 wherein the said organic mixture comprisesbutadiene and butene.

2. The process of claim 1 wherein the pressure on the second side of themembrane is less than atmospheric pressure and lower than the pressureon the other side of the membrane.
 3. The process of claim 1 wherein thesaid organic mixture is a liquid mixture.
 4. The process of claim 1wherein the said organic mixture comprises butadiene and butene.